System for gas processing

ABSTRACT

A power plant for the generation of electrical energy with a system ( 1 ) for processing flue gases resulting from a combustion of fossil fuels comprises, according to the invention, an adiabatic compressor ( 5 ) for a first low-pressure compression of the flue gases and a second multi-stage, low-pressure flue gas compression system ( 14 ) and a multi-stage, high-pressure CO 2  compression system ( 15 ), where both the low-pressure flue gas compression system and the high-pressure CO2 compression systems are combined in one single machine (C 2 ) and are arranged on one common shaft ( 16 ) driven by one common driver ( 17 ). A heat exchanger ( 8 ) facilitates an improved recovery of heat resulting from the cooling of the adiabatically compressed flue gases. The invention allows an improvement of the overall power efficiency of a power plant integrated with this processing system as well as a reduction of investment cost.

TECHNICAL FIELD

The present invention relates to systems for processing gas resultingfrom fossil fuel fired power plants for the generation of electricenergy. It relates in particular to a system for gas processing topurify such gas in order to facilitate the transport and storage ofcarbon dioxide.

BACKGROUND ART

In view of reducing the emission of the greenhouse gas carbon dioxide(CO2) into the atmosphere, the flue gases of fossil fuel fired powerplants for the generation of electrical energy are typically equippedwith so-called CO2-capture systems. CO2 gases contained in the fluegases is first separated, then compressed, dried, and cooled and thusconditioned for permanent storage or a further use such as enhanced oilrecovery. For safe transport, storage or further use, the CO2 isrequired to have certain qualities. For example, for enhanced oilrecovery the gas is to have a CO2 concentration of at least 95%, atemperature of less than 50° C. and a pressure of 13.8 Mpa. Flue gasesfrom fossil fuel fired power plants comprise not only CO2 but also anumber of further contaminants such as water vapor, oxygen, nitrogen,argon, as well as SO3, SO2, NO, NO2, which must be removed in order tofulfill the environmental regulations and requirements for transport andstorage of CO2. All of these contaminants and the CO2 itself can appearin various concentrations depending on the type of fossil fuel,combustion parameters, and combustor design. The percentage of CO2contained in the flue gases can range from 4% in the case of combustionof gases for a gas turbine to 60%-90% in the case of a coal fired boilerwith air separation unit providing additional oxygen to the combustionprocess. The removal of contaminants from flue gases is not limited bytechnical barriers but rather by the additional cost and energyrequirements and subsequent reduction in the overall power plantefficiency.

Minish M. Shah, “Oxyfuel combustion for CO2 capture from pulverized coalboilers”, GHGT-7, Vancouver, 2004, discloses an example of a system forhandling the flue gases resulting from a fossil fuel fired boiler. Thesystem includes a recycle line for a portion of the flue gas to bereturned to the coal-fired boiler together with oxygen from an airseparation unit. The flue gas is led through a filter for removal of ashand dust, such as a fabric filter or electrostatic precipitator,furthermore through a flue gas desulphurization unit for the removal ofSOx and finally through a gas processing unit for CO2 purification andcompression. This unit comprises a system for removal of incondensablegases such as O2, N2, and Ar, a dehydration system for removal of watervapor, and a series of compression and cooling systems. These include afirst low-pressure compression systems of the non-purified flue gasesand a high-pressure compression system of the purified CO2, each withcoolers integrated.

For the compression, such systems comprise for example two multistagecentrifugal compressors, a low-pressure compressor and a high-pressurecompressor and apparatuses for dehydration and cryogenic removal ofinert gases arranged between the low- and high-pressure compressors. Themultistage centrifugal compressors have intercoolers following eachcompressor stage in order to minimize the power consumption of thecompression. The multistage centrifugal compression typically includes4-6 compression stages. Because of the large number of compressorstages, the low-pressure and high-pressure compressors are each arrangedon independent shafts with a separate driver. The heat resulting fromthe intercoolers is low-level heat of 70-80° C., which is typically notrecovered but instead dissipated in the cooling water system of thepower plant. The cryogenic system for removal of inert gases generatesan inert gas flow under pressure, which is typically expanded in asuitable turbine, which in turn drives a generator or is arranged toprovide a part of the mechanical power for driving a compressor.

Furthermore, Bin Xu, R. A. Stobbs, Vince White, R. A. Wall, “Future CO2Capture Technology for the Canadian Market”, Department for BusinessEnterprises & Regulatory reform, Report No. COAL R309, BERR//Pub, URN07/1251, March 2007, discloses on pages 124-129 a system for processingthe flue gases including dehydration, compression, cooling, andcryogenic processing. The compressors used are adiabatic compressors,which allow an improvement in terms of power consumption and coolingrequirements.

U.S. Pat. No. 6,301,927 discloses a method of separating CO2 from a feedgas by means of autorefrigeration, where the feed gas is firstcompressed and expanded in a turbine. The CO2 contained in the feed gasis then liquefied and separated from its gaseous components in avapor-liquid-separator.

U.S. Pat. No. 4,977,745 discloses a method for recovering low purity CO2from flue gas including compressing flue gas and directing it through awater wash and a dryer and finally to a CO2 separation unit.

U.S. Pat. No. 7,416,716 discloses a method and apparatus for purifyingcarbon dioxide, in particular for the removal of SO2 and NOx from CO2flue gas resulting from a coal fired combustion process. For this, theflue gas or raw CO2 gas is compressed to an elevated pressure by meansof a compression train with intercoolers for the cooling of thecompressed gas, where some of the compression is performedadiabatically. The compressed gas containing water vapor, O2, SOx, andNOx is then led into a gas/liquid contact device for washing the gaseousCO2 with water for the removal of SOx and NOx.

SUMMARY OF INVENTION

In view of the described background art, it is an object of theinvention to provide a fossil fuel fired power plant for the generationof the electrical energy with an improved flue gas processing system forthe processing of the flue gases resulting from the combustion of thefossil fuel for the power plant.

According to the invention, a fossil fuel fired power plant comprises apost-combustion flue gas processing system, where the system comprises

a first low-pressure flue gas compressor, where the first low-pressureflue gas compressor is an adiabatic, axial compressor withoutintercooling,

one or more heat exchangers arranged downstream from the firstlow-pressure flue gas compressor and configured and arranged for thetransfer of heat from the compressed flue gas to the power plant or asystem connected with the power plant,

a second low-pressure flue gas compressor arranged downstream of the oneor more heat exchangers and having one or more stages and one or morecoolers,

a unit for cryogenic purification of the flue gases by removal of inertgases from the flue gas arranged downstream of the second low-pressureflue gas compressor, and

a high-pressure CO2 compressor system arranged downstream of the unitfor cryogenic purification and configured and arranged for thecompression of a CO2 flow resulting from the unit for cryogenicpurification, the high-pressure CO2 compressor system having severalstages and one or more coolers,

where both the second low-pressure flue gas compressor and thehigh-pressure CO2 compression system are combined in one single machineand are arranged on one common shaft that is driven by one commondriver.

The power plant with the post-combustion flue gas processing systemaccording to the invention allows, due to the integration of anadiabatic compressor, a reduction of the total power consumptionnecessary for the flue gas compression. Furthermore, the adiabaticcompressor without intercoolers allows a recovery of the heat from theflue gas and its use in the power plant or in a system connected withthe power plant such as an industrial consumer or other consumerrequiring heat. Thereby, required heat, for example for feedwaterpreheating, that would otherwise be extracted from the power plant cannow be drawn from the compressed flue gases. The system according to theinvention therefore facilitates an improvement in the overall efficiencyof the power plant thus integrated with the flue gas processing system,however without an increase in number of compressor machines.

Additionally, a flue gas processing system according to the inventionallows a reduction in the initial investment cost for the system. Thesystem comprises a total of only two compression machines with twodrivers and two shafts, i.e. the adiabatic, flue gas compressor on onehand and the combination of second low-pressure flue gas compressor withhigh-pressure CO2 multi-stage compressor, on the other hand. In spite ofthe addition of an adiabatic compressor, the system's total number ofmachines is still the same. Finally, the combination of the secondlow-pressure flue gas compressor and high-pressure CO2 compressor intoone machine results not only in a reduction in investment cost but alsoallows space efficiency in the power plant construction.

In a particular embodiment of the invention, the second low-pressureflue gas compression system and the high-pressure CO2 compression systemcombined into one machine arranged on one shaft comprises twolow-pressure compressor stages and four to six high-pressure compressorstages.

In a further particular embodiment of the invention, the flue gasprocessing system comprises a dehydration unit arranged downstream ofthe second low-pressure flue gas compressor. This allows greaterpossibilities in the handling and use of the resulting CO2.

In a further particular embodiment of the invention, the flue gasprocessing system comprises one or more heat exchangers for cooling ofthe flue gas downstream from the adiabatic compressor, where the heatexchanger(s) is/are configured for heat exchange with a water flow thatcan be part of the water/steam cycle of a power plant or any other waterflow system for heat recovery within the power plant or in a systemconnected with the power plant. For this embodiment, the adiabatic fluegas compressor is configured for a discharge pressure of the flue gasesof a selected pressure range. This pressure range is selected forexample in consideration of an optimal heat recovery in connection withthe water/steam cycle of the power plant, an optimally minimized powerconsumption of the adiabatic compressor, and the integration of the low-and high-pressure compression stages downstream from the adiabatic fluegas compressor.

In an embodiment, the adiabatic flue gas compressor discharge pressurecan be set to 7 to 9 bar abs. Above this pressure range the adiabaticcompression would require more power consumption than the compression inan intercooled centrifugal compressor. With this discharge pressure thetemperature at the discharge of the adiabatic compressor is in the rangefrom 170 to 280° C. This allows an efficient heat recovery for instanceby heating condensates from the power plant steam/water cycle throughthe use of a dedicated heat exchanger.

After the heat recovery, the flue gas is at a temperature of about 50°C. It is then further cooled in a second exchanger, where heat isdissipated. It is then compressed to 30 to 40 bar abs by two stages ofthe second low-pressure flue gas compressor, a centrifugal compressorwith intercoolers. These two stages can be easily combined with thehigh-pressure CO2 compressor having 4 to 6 stages, for instance by theuse of one integral gear compressor with 6 to 8 stages. The adiabaticcompressor facilitates an improved recovery of the heat resulting fromthe cooling of the compressed flue gas. This can further improve theoverall efficiency of a power plant integrated with this type of fluegas processing system. A further advantage of the power plant accordingto the invention is in that the number of flue gas compressors, thesebeing adiabatic and centrifugal, remains constant compared to powerplants of the prior art having only centrifugal compressors.

In a further particular embodiment of the invention, the first,low-pressure flue gas compressor and second low-pressure flue gascompressors are configured such that the ratio of the discharge pressureof the adiabatic compressor to the discharge pressure of the first stageof the low-pressure flue gas compressor is in the range from 1.5 to 2.5.

The power plant can be any kind of fossil fuel fired power plant,including a steam turbine power plant with a coal-fired boiler, wherethis boiler can be operated with or without additional oxygen providedby an air separation unit. The fossil fuel fired power plants can alsoinclude gas turbine or combined cycle power plants.

In a further embodiment, the system according to the invention furthercomprises a system for the removal or reduction of the SOx and NOx. Suchsystem can be arranged either in the low-pressure flue gas treatmentsystem, that is upstream of the flue gas compression or downstream fromthe adiabatic compressor. If the SOx and NOx removal system is arrangeddownstream from the adiabatic flue gas compressor, the proposedinvention can still be realized by combining the remaining centrifugalstages required for flue gas compression with the stages required forCO2 compression in one machine driven by one driver. The SOx and NOxremoval reaction kinetics as well as reactor sizing will affect thechoice of the adiabatic compressor discharge pressure. For instance, thedischarge pressure can then be raised to around 15 bar abs, thus leavingone stage of flue gas compression to be combined with the CO2compression in one multistage centrifugal compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of an embodiment of a flue gas processing systemaccording to the invention that may be integrated in a power plant forthe generation of electricity.

BEST MODES FOR CARRYING OUT THE INVENTION

FIG. 1 shows a flue gas processing system 1 for the processing of fluegases resulting from a fossil fuel fired power plant. The power plantitself is not shown save for a line 2 directing the flue gas resultingfrom the combustion of fossil fuels for the generation of a workingmedium to drive a turbine. The processing system 1 comprises essentiallya flue gas line 2, directing flue gases to a first compressor system C1,heat recovery system HR, a second compressor system C2, all arranged inseries in the sequence mentioned, and a CO2-line 3 for directing theseparated CO2 to a facility for further use. The flue gas line 2 leadsfrom a power plant to the first compressor system C1, which comprises anadiabatic flue gas compressor 5. The heat recovery system HR comprisesheat exchangers for the cooling of the compressed flue gases released bythe compressor C1 and transfer of heat from the flue gases to the powerplant. The second compressor system C2 comprises a combined multi-stageand intercooled compressor system for the low-pressure compression offlue gases and the high-pressure compression of purified CO2. Finally,the line 3 leads purified and compressed CO2 away from the system 1 to afurther system 4 for transport, storage or further use of the CO2 suchas enhanced oil recovery.

Flue gases are led to system 1 as shown via the line 2, where the fluegases can result for example from a coal-fired boiler, from a gascombustion chamber, or oxyfired coal-fired boiler. As such, they cancontain CO2 gas of various concentrations, such as 4% or more in thecase of a gas turbine power plant with or without flue gasrecirculation, or up to 60-90% in the case of oxyfired coal burningboilers for steam turbine power plant. Following the boiler orcombustion chamber, the flue gases may have been pre-treated in a filtersuch as an electrostatic precipitator or a fabric filter or any otherprocess unit for the removal of sulphur. Furthermore, the flue gases mayhave been treated in an apparatus for the removal of NOx or mercury.

The flue gas line 2 carries the CO2-containing flue gas to thelow-pressure, adiabatic flue gas compressor 5 driven by a driver 6 andconfigured to compress the flue gas to a discharge pressure of 5 to 20bar abs. A minimized power consumption for the compression can bereached with a configuration for a discharge pressure of 5 to 8 bar abs,for example 7 bar abs. The adiabatic compressor 5 is configured for acompression to a discharge pressure of no more than 20 bar. Compressionto a discharge pressure higher than this limit would increase the powerconsumption such that there would no longer be any benefits from the useof an adiabatic compressor. This is due to the fact that after apressure of around 8 bar abs, the adiabatic (axial) power consumptionbecomes higher than that of an intercooled centrifugal compressor. Afterthis pressure the benefit of having more efficient wheels in the axialmachine is more than compensated by the increase of power consumptiondue to the gas temperature increase in the absence of intercooling. Atthe compressor discharge the compressed flue gas may have a temperatureof ca. 200° C.-280° C. The optimum discharge pressure of the adiabaticcompressor will be set by the minimization of power consumption, butalso by additional parameters such as water/steam cycle integration,intermediate removal of SOx and NOx if any, as well as machineselection.

A line 7 leads from the discharge of the low-pressure flue gascompressor 5 to a first heat exchanger 8, through which the compressedand hot flue gases flow in counterflow to a flow of water or anothercooling medium. The cooling medium is led from the heat exchanger 8 vialine 9 to a system for heat recovery in a system within the power plantor in a system connected with the power plant. The adiabatic/axial fluegas compressor 5 allows the recovery of heat from the flue gases at ahigher temperature (170-240° C.) compared to the case if a centrifugalcompressor were used instead in this position. This heat can beeffectively used in the power plant. For example, in the embodimentshown, the heat recovery system is the water/steam cycle 9 of a steamturbine system. In a particular example, this water flow is connected toa feedwater preheater or to the condensate extraction pump. A part ofthe condensates can be heated directly by the flue gas, thus by-passingthe low-pressure heaters. The steam consumption of the low-pressureheaters is reduced and, as a consequence, more steam is expanded in thecycle steam turbine and the plant can produce more electrical power. Dueto the use of the adiabatic/axial flue gas compressor a gain of the netpower output of the power plant of 0.5% to 1% can be achieved over thenet output of a power plant having only centrifugal flue gascompressors. The power plant according to the invention achieves agreater output although having the same number of compressor machines asa power plant with only centrifugal compressors.

After having passed through the heat exchanger 8, the flue gases have atemperature of for example 50° C. On the flue gas side, the heatexchanger 8 is connected via a line 10 to a further heat exchanger orcooler 11, where the flue gases are further cooled to a temperature offor example 30° C. The heat resulting from this cooling is of low-gradeand can be dissipated.

A line 13 leads from the cooler 11 to the combined compression system C2driven by driver 17 and comprising a low-pressure flue gas compressor14, a high-pressure CO2 compressor 15 arranged on shaft 16 and driven bydriver 17. The low-pressure flue gas compressor can have for example twostages of a centrifugal compressor with intercooler, whereas thehigh-pressure CO2 compressor can have for example four to six stageswith intercoolers. If the discharge pressure of the adiabatic compressoris lower, that is within the discharge pressure range given between 5 to20 bar abs, the centrifugal low-pressure flue gas compressor can alsohave three instead of two stages. The flue gases, compressed to apressure of for example 30 bars abs by the low-pressure compressor 14,are led via line 18 to a dehydration unit 19 and thereafter to acryogenic unit 20. In the cryogenic unit, the flue gas is separatedresulting in a purified CO2 gas flow and a vent gas containing inertgases like nitrogen, oxygen and argon. The vent gas is sent via line 21to an expander 22, which can be mounted on the same shaft 16 or mountedon an independent shaft. In the flue gas processing system according tothe invention, the low-pressure flue gas compression system 14 andhigh-pressure CO2 compression system 15 are arranged on the same shaft,whereas the low-pressure flue gas compression system is arrangedup-stream of the cryogenic purification system and the high-pressure CO2compression system is arranged down-stream from the purification system.

The cryogenically purified flue gas, now containing mainly CO2 of aconcentration sufficient for transport and storage, is led from the unit20 to the high-pressure compressor system 15 for further compression toa pressure of 110 bar abs, from where it is finally led via line 3 to asystem 4 for further use of the CO2. The cryogenic process can beoptimized in that the purified CO2-gas is fed in two separate flows tothe compressor system 15 at two different pressures respectively, bywhich the compressor power consumption is minimized. One firstlow-pressure line feeds the purified CO2 gas to the front inlet of thecompressor system 15 and a second medium pressure line feeds thepurified CO2 gas to an intermediate stage of the compressor system 15.

TERMS USED IN FIGURES

1 system for processing flue gases

2 flue gas line from power plant

3 line for purified CO2 gas

4 system for transport, storage or further use of purified CO2

5 adiabatic compressor

6 driver

7 flue gas line

8 heat exchanger

9 system for cooling medium

10 flue gas line

11 heat exchanger

12 system for cooling medium

13 flue gas line

14 low-pressure compressor for flue gas

15 high-pressure compressor for CO2 gas

16 shaft

17 driver for combined low- and high-pressure compressor

18 flue gas line

19 dehydration unit

20 cryogenic unit

21 line for inert gases

22 expander for vented inert gases

C1 adiabatic compressor

C2 combined compressor machine

HR heat recovery system

1. A system for processing flue gases from a fossil fuel fired powerplant for the generation of electrical energy comprising: an adiabaticcompressor for a first low-pressure compression of the flue gas; asecond low-pressure compression system having one or more stages and oneor more coolers; and a high-pressure compression system having severalstages and one or more coolers, where both the second low-pressurecompression system and the high-pressure compression system are combinedin one single machine, and arranged on one common shaft, and driven byone common driver.
 2. The system according to claim 1, furthercomprising a unit for cryogenic purification of the flue gases byremoval of inert gases from the flue gas, where the unit for cryogenicpurification is arranged downstream of the second low-pressurecompression system and upstream of the high-pressure compression system.3. The system according to claim 1, further comprising a dehydrationunit arranged downstream of the second low-pressure compression system.4. The system according to claim 1, wherein the system comprises twolow-pressure compressor stages and four to six high-pressure compressorstages arranged on one single shaft.
 5. The system according to claim 1,further comprising a heat exchanger arranged downstream from theadiabatic compressor.
 6. The system according to claim 1, furthercomprising a heat exchanger configured for heat exchange with a waterflow system for heat recovery.
 7. The system according to claim 1,further comprising a heat exchanger configured for heat exchange with awater flow system for heat recovery where the water flow system is partof a water/steam cycle of a steam turbine power plant.
 8. The systemaccording to claim 1, further comprising a water flow system isconnected to a condensate extraction pump.
 9. The system according toclaim 1, wherein the adiabatic compressor is configured for a dischargepressure of the flue gases of a pressure in a range from 5 bar abs to 20bar abs.
 10. The system according to claim 1, wherein the adiabaticcompressor is configured for a discharge pressure of the flue gases of apressure in a range from 7 bar abs to 9 bar abs.
 11. The systemaccording to claim 1, wherein the adiabatic compressor and low-pressurecompression system are configured such that a ratio of a dischargepressure of the adiabatic compressor to a discharge pressure of a firststage of the low-pressure compression system is in a range from 1.5 to2.5.
 12. The system according to claim 1, further comprising a firstline for low-pressure purified CO₂ gas which leads from a cryogenicpurification unit to a first inlet of the high-pressure compressionsystem, and a second line for medium-pressure purified CO₂ gas whichleads from the cryogenic purification unit to an intermediate stage ofthe high-pressure compression system.
 13. The system according to claim1, further comprising a system for the removal or reduction of SOx andNOx arranged either in a low-pressure flue gas treatment system upstreamof the flue gas compression systems or after the adiabatic compressor.14. The system according to claim 1, wherein the system is integratedwith a power plant fired by gas, coal, oxyfired coal, or a gas turbinepower plant with a facility for post-combustion CO₂-capture.